The present invention relates to oxide particles having a satisfactory particle size and a satisfactory shape and also having high piezoelectric properties; a piezoelectric element produced using the oxide particles; and a method for producing the oxide particles.
As a piezoelectric material having excellent piezoelectric properties, PZT having a compositional formula of Pb(ZrxTi1-x)O3 has been known.
PZT has been used in various applications, such as piezoelectric actuators, piezoelectric sensors, piezoelectric buzzers, acceleration sensors, and power generation element, by utilizing its excellent piezoelectric properties.
As methods for producing such a PZT, methods by which a piezoelectric ceramic is formed by mixing raw material powders such as lead zirconate and lead titanate and then sintering, and methods using hydrothermal synthesis have been known.
As an example of the methods for producing PZT using hydrothermal synthesis, a method described in R. E. Riman et al., Langmuir 2005, 21, 3207-3212 has been known.
In this method for producing PZT, first, a raw material in which titanium oxide powder and zirconium oxychloride are dispersed in water, and another raw material in which lead acetate powder and EDTA are dispersed in water and the pH has been adjusted by potassium hydroxide are prepared. Both of the prepared raw materials are then mixed, and the mixed raw material is subjected to hydrothermal synthesis using an autoclave to produce PZT powder.
According to the method for producing PZT described in R. E. Riman et al., Langmuir 2005, 21, 3207-3212, PZT particles having a uniform size and shape that are in a several μm cube shape can be produced.
Note that, in piezoelectric materials, an MPB composition has been known as a composition that can provide high piezoelectric properties. The MPB composition of PZT is Pb(Zr0.52Ti0.48)O3.
On the other hand, the composition of PZT particles produced by the production method described in R. E. Riman et al., Langmuir 2005, 21, 3207-3212 is Pb(Zr0.7Ti0.3)O3. Because of this, the PZT particles obtained by the production method described in R. E. Riman et al., Langmuir 2005, 21, 3207-3212 have piezoelectric constant d33 of approximately 80 to 120 pm/V, and thus high piezoelectric properties cannot be achieved.
Furthermore, when PZT of the MPB composition is produced using the production method of R. E. Riman et al., Langmuir 2005, 21, 3207-3212, the shape or size of the particles cannot be controlled, and thus PZT particles having a uniform size and shape, similarly to those with the composition of Pb(Zr0.7Ti0.3)O3 described above, cannot be obtained.
Meanwhile, R. E. Riman et al., Journal of Crystal Growth 226 (2001) 313-326 describes the method for producing PZT particles of the MPB composition via hydrothermal synthesis.
In this production method, first, zirconium propoxide and titanium isopropoxide are added to propanol to coprecipitate, thereby producing a zirconium titanium oxide precursor (ZTO precursor). Thereafter, lead acetate, TMAH, and the ZTO precursor are added to water and subjected to hydrothermal synthesis to obtain PZT particles.
According to the method for producing PZT described in R. E. Riman et al., Journal of Crystal Growth 226 (2001) 313-326, PZT particles in a several μm cube shape of the MPB composition can be obtained.
However, PZT particles produced by this method have an uneven cube shape and size variation is also large, compared to PZT particles produced by the method described in R. E. Riman et al., Langmuir 2005, 21, 3207-3212. Furthermore, this production method requires a complicated two-step process in which the ZTO precursor is produced and then subjected to hydrothermal synthesis.
Furthermore, the PZT particles obtained by this method for producing PZT have high surface porosity (porous area on the surface/surface area); and, because of this, high piezoelectric properties cannot be achieved.
As is well known, piezoelectric materials exhibit piezoelectricity when subjected to heat treatment.
Note that, the particle with high surface porosity has a large surface area and, furthermore, the porous part is considered to be defective as a structure. In PZT ceramics, volatilization of lead during firing is typically problematic, and, similarly, PZT particles having high surface porosity also readily allow volatilization of lead.
Because of this, when PZT particles having high surface porosity are heat-treated, the compositional ratio of lead to zirconium and titanium (Pb/(Zr+Ti)) in the composition after heat treatment becomes less than the compositional ratio before the heat treatment.
In PZT ceramics or the like, when the amount of lead is low (when 1>Pb/(Zr+Ti)), piezoelectric properties are typically lowered. That is, PZT having high surface porosity results in a condition where the amount of lead is insufficient due to the heat treatment, and thus tends to exhibit lower piezoelectric properties.
Meanwhile, for piezoelectric ceramics for which sintering is performed, a larger amount of lead is used, in a manner such that 1≤(Pb/(Zr+Ti)) is satisfied, in a raw material condition (preparation) before sintering to prevent the lead amount from being insufficient.
However, in production of PZT via hydrothermal synthesis, a PZT particle is a particle that is similar to a single crystal. Because of this, PZT particles produced by hydrothermal synthesis are less likely to have a region where excessive lead is absorbed, such as grain boundary in the particles, and thus excessive lead is less likely to be absorbed in the particle. As a result, even when hydrothermal synthesis is performed under a condition where excessive lead is used in the raw material stage, the composition of the synthesized PZT particles results in approximately 1≈(Pb/(Zr+Ti)). In particular, as the PZT particles are in shapes closer to a cube shape, this tendency is stronger since the PZT particles are closer to single crystals and less likely to have grain boundary.
Therefore, PZT particles having high surface porosity allow a larger amount of lead to volatilize during heat treatment. As a result, the particles after the heat treatment tend to have insufficient amount of lead, and high piezoelectric properties is less likely to be achieved even when the particle has the MPB composition.